1. Field of the Invention
The present invention relates generally to measurement techniques for performing adsorption and desorption gas analyses on materials. By employing the apparatus and method disclosed herein, the void volume is determined without the use of a non-adsorbable gas, such as helium. Unlike previous methods and apparatus, the adsorption of the sample cell walls are measured. By using the same gas that is used to calibrate the sample cell for measuring the adsorption and desorption properties of the samples, pressure--volume data points can be used to prepare adsorption and desorption isotherms, BET surface area, and other relevant information.
2. Description of the Prior Art
A great variety of applications in modern technology require accurate measurements concerning the microstructure of materials, such as powders. These materials are widely used, for example, as catalysts, or in the production of paint, cement, carbon blacks, absorbents, dessicants, and the like. The information desired regarding the microstructure of these materials includes the porosity and surface area of the powder as well as the distribution of pore volume in the various sized pores.
Many types of apparatus and systems have been developed for performing adsorption and desorption measurements on samples. All of these adsorption and desorption measurement systems have certain essential features. These include a non-adsorbable gas (e.g. helium) for calibrating the volume of the cell containing the sample, and an adsorbate gas for performing the adsorption analysis. The non-adsorbable gas is critical in these systems for calibrating the volume of the sample cell with the sample material present.
Errors in the existing methods of measurement arise principally from three sources. First, because a non-adsorbable gas is being used to calibrate the volume of the sample cell, the adsorption of the sample cell is not measured. Thus, when the adsorbate gas is subsequently introduced into the sample cell when it contains the sample, all of the gas adsorbed is assumed to be adsorbed by the sample itself. Any gas adsorbed by the cell walls is not considered. Therefore, the adsorption of the cell is not compensated for when measuring the adsorption of the sample. While this may be negligible when the surface area of the sample is high, the error increases as the surface area of a sample decreases and the amount of gas adsorbed by the cell wall is a greater fraction of the total gas adsorbed.
Second, when the sample cell is immersed in a coolant (for example, liquid nitrogen) often required for adsorption and for maintaining a constant temperature of the sample during the measurement process, the attained temperatures cause the adsorbate gas to deviate from the ideal gas law (PV=nRT). Therefore, the gas contained in the void volume of the sample cell which is cooled, must be corrected for deviations from ideality. However, because measurements are time consuming and must be made with precision, and because the void volume of the cell is not measured as accurately as desired, the amount of correction for the volume of the gas under non-ideal conditions is indeterminate. Attempts to resolve this problem in the past have focused upon minimizing, as much as possible, the amount of void volume in order to reduce the amount of error in the measurements. This has met with varying degrees of success.
The third source of error results from that portion of the sample cell located above the immersion bath. This portion of the sample cell is in a transition zone of non-uniform temperature ranging from the temperature of liquid nitrogen to the ambient temperature. This contributes to an ambiguous temperature zone which cannot be precisely determined and thus contributes to a further degree of error.
Further problems involving speed, cost, and difficulty in processing are related to using a non-adsorbable gas. Non-adsorbable gases, for example, helium, are expensive and add the need for a second gas with which to work. Additionally, they are expensive per unit volume, thereby increasing the cost of each measurement. Further, apart from not measuring the adsorption of the walls of the sample cell during calibration, it is time consuming to determine the volume of the sample cell each and every time a sample is to be measured in that sample cell.
There is therefore a great need in the art to correct for these major sources of errors in measuring the adsorption and desorption of a sample. Further, a method and apparatus to avoid these aforementioned sources of error and difficulties of measurement and to do so by using only the adsorbate gas itself has been heretofore unrealized.
Accordingly, there is now provided with this invention an improved apparatus and method for measuring the void volume, and the adsorption of the sample cell walls, and correcting for non-ideal gas behavior by using only the adsorbate gas. This present invention effectively overcomes the aforementioned difficulties and longstanding problems inherent in surface area and pore volume measurement. These problems have been solved in a simple, convenient, and highly effective way by which to increase the accuracy of the measurement and to decrease the time consumed in performing the measurement. Additional objects of the present invention will become apparent from the following description.